Best way of connecting two meshes, one with many faces
A typical subdivision approach would avoid using meshes of different densities where possible, and rely on Catmull-Clark subdivision to create curvatures/circles..

Any (6+)-gon will make a reasonable circle under subdiv. Often made from 4 quads:


Select all quads
CtrlX Dissolve central vertex
AltP Poke single face
-CtrlShiftB Bevel central vertex
Shipped Add-On, Loop Tools > Circle the result




Then, with extrusions, and some rotations/scales around Custom Orientations, you can model something like this mask without having to reduce topology to match between densities.



If you do really need to match areas of significantly different detail, often, to avoid fiddly hand work, you can model at a lower level of subdivision than your intended result, apply one level of subdiv, and continue modeling from there.